Induction brazing uses the same basic principles as an induction hob you may find in your kitchen or employed in heat-treating steel. The article which follows will look into the benefits and limitations of induction brazing as well as examine the principles and best practices to ensure a sound braze joint.
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Principle of Induction Heating
Induction heating works on the theory that eddy currents are generated within electrically conductive materials by applying electromagnetic induction. These eddy currents coupled with the electrical resistance within the material gives rise to Joule heating. The induction heater consists of an electromagnet through which a high frequency alternating current is passed. The frequency of the AC current is just one of the important factors in determining the degree of heating and the depth of penetration of the heat.
What Materials Can Be Brazed?
Induction heating works on electrically conductive materials, usually this limits the process to metals but there are ways of circumventing these limitations. The ease with which a material can be heated by induction depends on the electrical resistivity and their magnetism. Ferrous metals are therefore much easier to heat than aluminium. Likewise carbon steels will heat more readily than stainless steels. Ceramics are not usually brazed by induction but if they are being brazed to a ferrous metal then heat transfer via conduction into the ceramic may allow a user to tackle this challenge.
The Equipment
Induction brazing equipment consists of a generator which is rated by power and frequency and an induction coil which is usually water-cooled. There will be fixturing to hold parts and depending on the application, a facility to shroud the components in an inert gas to prevent oxidation. If the process is automated the physical size can be significant although smaller table top versions are also available on the market.
Induction Brazing Parameters
What else needs consideration?
Temperature control
Optical pyrometers are the most effective way to control temperature during induction brazing. A feedback control loop integrated with the induction generator allows instant and accurate control. If a shroud is to be used around the component to permit the use of an inert or oxidizing atmosphere then the pyrometer must have visibility of the braze joint. This can be facilitated by using quartz glass which allows the pyrometer to read the temperature accurately.
Fixturing
Any fixturing within 50mm of the coil could be affected by the electromagnetic current and so should be made from non-magnetic materials such as aluminium or austenitic stainless steel. Ceramics are often used as bases for supporting components.
Automating the process
Being such a repeatable process, induction brazing lends itself to automation. Carousels, pick and place, turntables can all be incorporated to automate or semi-automate the process.
Pros and Cons of Induction Brazing
Benefits:
Limitations:
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Typical Applications
Case Study: Joining tungsten carbide to tungsten carbide using a Palladium based braze alloy (VBC Alloy ) with a liquidus temperature of °C.
The surfaces to be joined are first carefully cleaned in an ultrasonic bath containing acetone. The braze media is a foil preform sized to ensure complete coverage of the joint once molten and is likewise cleaned. The parts are pre-assembled and placed into a fixture that applies the correct pressure to the components.
The fixture is enclosed in quartz glass and allows a Nitrogen/Hydrogen atmosphere to flow over the parts during the induction heating cycle. The induction coil is designed to ensure even heating throughout the joint. The gas flows over the assembly prior to the initiation of the heating cycle and remains on until the assembly has cooled to a safe temperature where oxidation will not occur.
After cooling the braze foil can be seen to have fully wet the surfaces forming a perfect bond and the external surface of the tungsten carbide has been cleaned due to the reducing effect of the hydrogen.
Case study: Joining copper to carbon steel &#; dissimilar materials. One of the problems engineers will come across frequently is the difference in coefficients of thermal expansion when joining different materials.
In this case, a copper pipe of diameter 50mm with a CTE of ~16.5 ppm/°C to be joined to a carbon steel pipe (CTE ~11 ppm/°C) of similar diameter. The copper pipe is expanded allowing the steel pipe to slide in with a clearance of 0.1mm. This gap allows the braze alloy to flow into the joint. The filler metal to be used in this case is silver-based VBC Alloy (Bag-7) which has a brazing temperature of 675°C -760°C. It is used in wire formed into a ring which is placed at the joint.
A brazing flux (complying to EN FH10) is used to prevent oxidation during the heating cycle to allow the molten alloy to flow into the joint. This low temperature brazing alloy is a suitable alternative to cadmium containing alloys. Due to the ductile nature of silver/copper based alloys it will be capable of accommodating the difference in thermal expansion of the two parent metals.
The assembly is placed into a suitably sized induction coil which is powered by a 30KW generator. The frequency is set to 100 kHz. The cycle time is 60 seconds during which the components heat, the braze alloys melts and flows into the joint and is then allowed to cool.
After brazing the corrosive flux residue needs to be removed with hot water and mechanical brushing.
In summary, Induction Brazing should be considered as an attractive alternative to flame or furnace brazing due to the speed and relative low cost of the process. Please contact with any queries relating to Induction Brazing. See https://www.vbcgroup.com/brazing-materials for our full list of braze alloys.
Very Important &#; Keep the coupling-distance as even as possible around the braze-joint! It is ESSENTIAL that the coupling distance be carefully controlled during induction heating/brazing, since it will strongly affect the heating of the metal placed inside the induction coil. If the part being brazed is held too close to one side of the coil (tight coupling distance) and thus quite far from the other side of the coil (loose coupling distance), the heating of the metal part will not be uniform, and can easily and quickly result in overheating of the metal surfaces that have the tight coupling distance, leaving the surface with the loose coupling distance cool by comparison. This could easily result in uneven melting of any applied brazing filler metal (BFM), as well as uneven flow of the BFM through the joint.
Recommendation: It is probably best to use a semi-automated setup when induction brazing, so that the part to be brazed can be mechanically raised and lowered into and out of the coil, thus keeping the part accurately centered in the induction coil, with as even a coupling distance around the part as possible.
3. Frequency setting. The frequency setting of the induction machine, simply put, tells you how frequently the direction of the electrical flow is reversed. Whereas the alternating current (AC current) we use in our homes in the US may operate at a 60-cycle frequency, that of an induction brazing machine is much higher, on the order of tens of thousands of times per second, or higher (hundreds of thousands per second, and even millions of times per second).
It is interesting to note that the higher the frequency, the more intense will be the surface heating rate. This may, or may not, be good for brazing, depending on the thermal conductivity of the metal being joined. The frequency level chosen is best determined by experimentation, so as to optimize the frequency range best suited for your particular design and base metals being used.
4. Power of induction-machine. The larger the part that is to be brazed, the greater should be the available power in the induction machine in order to be able to heat that greater mass in a reasonably short period of time. For small parts, a one-kilowatt (1-kw) or a 5-kw desk-top machine may be quite adequate, allowing parts to come up to temperature in a matter of seconds. Typical induction brazing cycles might range from about 15-seconds all the way up to a few minutes. As you can see, the induction-brazing cycle time is much shorter than would be the case if the same parts were being furnace-brazed.
For induction-brazing of large parts, large power units that sit on the floor may be needed. Such units may have power ratings from about 10-kw up to 50-kw or higher in order to be able to supply sufficient electrical power for large brazing needs.
5. Coil Design. Designing an appropriate coil for the part you wish to braze is not a simple matter. In my opinion, you should always have the induction-machine manufacturer (from whom you purchased the equipment) design and build the induction-brazing coils for you. That is their job! Do NOT merely buy the equipment from the manufacturer and then try to make your own induction-brazing coil. That is truly being &#;penny wise and pound foolish&#; as the British phrase goes. Study some of the coil-designs shown in Fig. 6.
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